Phase-locked loop (PLL) frequency synthesis is a well-known technique for generating a variety of signals of predetermined frequency in many applications, e.g., digital radiotelephone systems. Briefly, the output of a voltage-controlled oscillator (VCO) is coupled to a frequency divider for providing one input to a phase detector. Another input to the phase detector is a reference signal from a fixed frequency source having high stability over a range of operating conditions. Differences in phase determined by the phase detector (typically reflected as charge pulses) are then filtered and applied to the VCO to control changes to the frequency of the VCO of such magnitude and sign as to reduce the detected phase difference.
Fractional-N (F-N) synthesizers based on the above-described PLL frequency synthesis techniques have been in favor for some time because, inter alia, they provide for non-integer division of the VCO output, thereby providing greater flexibility in choosing VCO outputs, and allowing the use of higher frequency reference sources with the concomitant potential for wider bandwidth and faster loop locking times. Other aspects of F-N synthesizers are presented in incorporated patent application (i) cited above.
It is often necessary in radiotelephone systems to apply modulation to a synthesized carrier to generate a modulated carrier. In some applications it has proven useful to apply modulating signals to modify the value of a frequency divider in PLL synthesizers (including F-N synthesizers) to derive the desired carrier modulation. Some radiotelephone systems employ so-called I-Q modulators to impart modulation information to transmitted carrier signals. In such schemes digital data are typically converted into I and Q (in-phase and quadrature) analog signals that are applied to respective mixers, whose outputs are then combined to form a composite modulated signal. This composite signal is then mixed to the desired output frequency.
Gaussian Minimum Shift Keying (GMSK) modulation is an I-Q modulation technique used in many radiotelephone systems, including widely deployed GSM mobile systems. Some GMSK systems provide digital I-Q modulation in a configuration generally of the form shown in FIG. 1. There, a first (IF frequency) VCO 107 is controlled in a PLL comprising stable frequency source (e.g., crystal) 100 to produce a reference frequency at the output of associated reference oscillator 101. The output of oscillator 101 is then conveniently divided in reference divider circuit 102 and applied as one input to phase detector 103. A second input to phase detector 103 is provided by loop divider 104 receiving the output from VCO 107. Phase detector 103 reflects any phase discrepancies between its inputs by supplying charge pump 105 with an appropriate pulse, which, after filtering in low-pass filter 106 is used to adjust the frequency of VCO 107. An output from VCO 107 is provided to quadrature network 109 for deriving respective I and Q components corresponding to the output of VCO 107.
Modulation inputs to the transmitter of FIG. 1 are presented in an illustrative parallel four-bit non-return-to-zero (NRZ) format to interface unit 110 for conversion to a serial format before being presented to phase mapping circuit 112. Mapping circuit 112 converts a serial input data stream into sequences of in-phase and quadrature phase pulses representative of the I and Q modulation components appearing on leads 113 and 114, respectively. In appropriate cases, mapping circuit 112 is realized as data-addressed I and Q read-only memories for producing input-data-controlled pulse sequences on respective circuit paths 113 and 114. These pulse sequences are then applied to respective digital filters 115 and 116, digital-to-analog converters (DACs) 117 and 118, and low pass smoothing filters 119 and 120 to provide analog pulses having shapes appropriate for QMSK modulation. See, for example, B. Razavi, RF Microelectronics, Prentice-Hall, 1998, especially pp. 150–152.
In many applications, relevant ones of circuit elements 110 through 120 will be found on a semiconductor chip that also includes a digital signal processor (DSP) or other source of modulating signals. In such cases serial interface 110 will not always be necessary, because the illustrative DSP (or other signal source) will provide modulating signals in appropriate form to drive phase mapping circuit 112 or equivalent functionality. In any event, elements 112 and 115 through 120 will advantageously function in close cooperation with a signal source (such as a DSP) to provide smoothed analog modulating signals at the outputs of filters 119 and 120.
Then, the smoothed I and Q pulse sequences are applied at respective mixers 121 and 122 to be combined with corresponding I and Q IF signals from VCO 107 via quadrature network 109. The mixed outputs from mixers 121 and 122 are then combined in well-known fashion in combiner 125, and, after IF bandpass filtering in filter 165, are applied to mixer 170, which also receives transmit carrier signals from VCO 160 connected in its associated PLL loop comprising loop divider 140, phase detector 135, charge pump 145 and loop filter 150. The second input to phase detector 135 is provided by oscillator 101 as modified by reference divider 130, as appropriate to particular frequencies employed. The finally mixed, GMSK-modulated carrier is further bandpass filtered in filter 180 before being applied to power amplifier 190 and thence to the transmit antenna.
As will be appreciated from a consideration of FIG. 1, prior art I-Q modulation techniques employ a variety of complex filtering, digital-to-analog conversions, and multiple PLL synthesizers necessitating complex circuitry and concomitant high power expenditure. Though direct digital interfacing to F-N synthesizers is possible, a predominant percentage of baseband modulation inputs presently available for use with F-N synthesizers (or other frequency sources) include only analog modulation inputs. Therefore, a digital modulator having reduced parts count, lower operating current and simplified operation, and which can accept analog I and Q data streams to control modulation in a F-N synthesizer is highly desirable. Moreover, modulators capable of accepting either analog or digital inputs are likewise desired in modulating F-N synthesizers.